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Gravity and Magnetic Expression of the San Leandro Gabbro with Implications for the Geometry and Evolution of the Hayward Fault Zone, Northern California

U.S. Geological Survey
345 Middlefield Rd., MS 989
Menlo Park, California, 94025
ponce{at}usgs.gov
(D.A.P., T.G.H., R.C.J.)

View larger version (105K): [in a new window]   Figure 1. Shaded relief topographic map of the Hayward Fault Zone and vicinity showing seismicity (circles). Seismicity data are from the U.S. Geological Survey's Northern California Seismic Net (NCSN) catalog from 1969 through 23 September 1999, M 1.0, P-wave arrivals 8, root-mean-square travel-time residual 0.3 sec, horizontal error 2.5 km, and vertical error 5.0 km. Circle, seismicity data (symbol size is proportional to earthquake magnitude); black lines, faults from Jennings et al. (1977) and recent trace of Hayward Fault from Lienkaemper (1992). AA', BB', and CC', geophysical profiles.

View larger version (92K): [in a new window]   Figure 2. Isostatic gravity map of the Hayward Fault Zone and vicinity. Black triangles, gravity stations; red lines, faults from Jennings et al. (1977); black line, recent trace of Hayward Fault from Lienkaemper (1992); white circles, seismicity as in Figure 1.

View larger version (64K): [in a new window]   Figure 3. (A) Magnetic map of the Hayward Fault Zone and vicinity composed of two aeromagnetic surveys flown in a northeast direction, with a flight-line spacing of 500 m ( mile) and a nominal flight-line elevation of 250 m (820 ft) above water and 300 m (1000 ft) above land. Explanation as in Figure 2. (B) Magnetic map enhancing shallow magnetic sources of the Hayward Fault Zone and vicinity. Shallow magnetic sources were enhanced by upward continuing the magnetic data a small distance (250 m) and then subtracting this field from the unfiltered data set to derive a residual field. The resulting map highlights magnetic sources within about 2.5 km of the surface. Explanation as in Figure 2. Expanded view of rectangular area is shown in Figure 4.

View larger version (56K): [in a new window]   Figure 4. (A) Magnetic map enhancing shallow magnetic sources of the San Leandro gabbro and vicinity. Patterned area, San Lenadro gabbro; red outlines, gabbro slivers defined by magnetic anomalies. Explanation as in Figure 2. Geology modified from Graymer (2000a). (B) Geophysical boundary of the San Leandro block and other geophysical lineaments defined by maximum horizontal gradients of gravity (blue) and magnetic (red) data. Bold blue and red lines define boundary of the San Leandro block. Geology modified from Graymer (2000a). Jgb (violet), gabbro; sp (blue), serpentinite; Tn (brown), Neroly Sandstone; black lines, faults. Bold black lines, recent trace of the Hayward and Chabot faults (Lienkaemper, 1992; Graymer, 2000a). Eastern edge of San Leandro block correlates with the Chabot fault.

View larger version (35K): [in a new window]   Figure 5. Gravity and magnetic models across the San Leandro gabbro. Physical properties are based on measurements on rocks and core samples (Table 1). Profile AA' is along and partly based on a geologic cross section by Graymer (2000a). Profile BB' and CC' are based on surface geology by Graymer (2000a). Geologic symbols: Qa, Quaternary alluvium and related low-density materials; Tn, Neroly Sandstone; Ts, undifferentiated Tertiary sedimentary rocks; KJf, Franciscan Complex; Kjm, Joaquin Miller Formation; KJk, Knoxville Formation; Ko, Oakland Conglomerate; Ks, undifferentiated Cretaceous sedimentary rocks; Jb, basalt; Jgb, gabbro; Jpb, pillow basalt; Jsv, quartz keratophyre; sp, serpentinite; sp-px, pyoxenite. Geographic and other symbols: SF Bay, San Francisco Bay; SL Res, San Leandro Reservoir; Inverted red triangles, recent trace of Hayward Fault (HF) and Chabot Fault (CF); D, density in g/cm3; S, susceptibility in CGS units; Drillhole symbol, thickness of offshore sedimentary basin from seismic and gravity data (Marlow et al., 1999); +, seismicity from relocated hypocenters using a high-resolution double difference relocation procedure within 5 km of the profile (Waldhauser and Ellsworth, 2000, 2002). No vertical exaggeration.

View larger version (59K): [in a new window]   Figure 6. Schematic cross sections along profile AA' across the northern part of the San Leandro gabbro. (A) Model of a tabular San Leandro gabbro body detached from oceanic basement layer; (B) model suggesting that the San Leandro gabbro is attached to flat lying oceanic basement; (C) model with the San Leandro gabbro and coast range fault folded and thrusted and extending to the Mount Diablo region. Geophysical modeling may not be able to resolve the differences between model A and B, but model C is unlikely. Geologic symbols: Qa, Quaternary alluvium and related low-density materials; Ts, undifferentiated Tertiary sedimentary rocks; KJf, Franciscan Complex; Ku, undifferentiated Cretaceous Great Valley Complex; Jgb, gabbro. Inverted black triangles, recent trace of Hayward Fault (HF) and Chabot Fault (CF); ?, questionable boundary, other boundaries based on geologic information and geophysical modeling.